Interestingly, a USN report from fairly early in the war with a rather small statistical sample drew the conclusion that the F4U was twice as likely to be lost to AAA fire than the AD's which were twice as likely to be lost to AAA fire as the F9F. A post WW2 USN statistical study also drew the conclusion that the F4U was particularly vulnerable to ground fire. The Korean War era report concluded that the problem lay on the oil cooling system's vulnerability.